Method of fluid flow control using a porous media

ABSTRACT

An apparatus for use in space environments to control the flow of a propellant to a thruster at a low mass flow rate comprises an insulated body having a cavity therein defining a vaporizing chamber, a porous medium having a low liquid permeability disposed in the chamber, a conduit means for feeding a propellant liquid into the chamber and exhausting a propellant vapor from the vaporizing chamber, and a heating means communicating with the medium within the chamber for controllable heating of the liquid propellant in the chamber creating a variable liquid/vapor transition zone within the medium. The transition zone is responsive to the heating means to control the mass flow rate of propellant through the vaporizing chamber. As the liquid/vapor transition zone is moved upstream by increasing the heater output, and thus increasing the volume of vapor within the porous medium, mass flow through the porous medium is decreased. Conversely, movement of the liquid/vapor transition zone downstream through the medium by decreasing the heater output, and thus exposing more of the porous medium volume to liquid, increases the mass flow rate.

This application is a division of pending application Ser. No.07/598,457, filed Oct. 12, 1990.

BACKGROUND OF THE INVENTION

This invention generally relates to flow control mechanisms and morespecifically to a fluid flow control apparatus preferably for use inspace to control the flow of propellant to an electrothermal thruster.

Electrothermal thrusters, such as arcjets or resistojets, are utilizedin space applications for attitude control and station keeping ofsatellites. These thrusters generally use a gaseous or vapor propellantfeed. Storage of propellants as liquids rather than as gases is moreattractive for technical and economic reasons. "Storable" liquidpropellants, such as water or ammonia, must therefore be vaporized onorbit prior to use in a cold gas or electric thruster.

Vaporization of a storable liquid propellant for electric propulsiondevices in the zero gravity environment of space is complicated by thefollowing:

1. Maintenance of good thermal contact between the liquid and theheating surfaces is not aided by gravity. Suitable heat transfercoefficients are therefore difficult to achieve.

2. Unstable or oscillatory flow can occur due to improper feedstreamimpedance. This problem is characteristic of the once through boilers(vaporizers) typical of space applications.

3. Variable control of flow rate, i.e. throttling, if desired, is verydifficult for the low flow rates typical of electric thrusters used forattitude control and station keeping such as resistojets and arcjets.For typical mass flow rates of less than 1 lbm/hr, liquid volumetricflow rates are so low that control valve resolution is inadequate. Vaporphase volumetric flow rates are higher, to be sure, but hot-gas flowcontrol valves are subject to material compatibility and reliabilityproblems which are not compatible with the high reliability and longlife requirements of space applications.

SUMMARY OF THE INVENTION

The invention disclosed herein addresses the problem of providing avariable control of mass flow rate at the very low rates required ofelectric thrusters, i.e. less than about one or two pounds mass per hour(lbm/hr). The apparatus in accordance with the present inventionaddresses primarily problem 3 above and in so doing facilitates asolution of problems 1 and 2 in one integral device. The apparatus isparticularly adapted to provide variable control of mass flow to anarcjet thruster. In so doing, the apparatus in accordance with thepresent invention permits the maintenance of good thermal contactbetween the propellant liquid and the source of heat causingvaporization of the liquid propellant. In addition, the presentinvention minimizes oscillatory flow due to improper feedstreamimpedance.

The method of propellant flow control in accordance with the presentinvention, simply stated, comprises the steps of:

(1) Providing a storable liquid propellant in a storage container;

(2) Providing a flow path for the liquid propellant from the storagecontainer containing the propellant to an electric thruster;

(3) Interposing a porous medium in the flow path which has a low liquidpermeability (as defined by Darcy's Law) so as to effectively restrictor limit the flow of the liquid propellant through the medium; and

(4) Controllably heating the medium to controllably vaporize at least aportion of the liquid propellant in the medium creating a back pressurein the medium to thereby control the rate of propellant fluid flow tothe electric thruster.

The apparatus necessary to carry out the method of the present inventionincludes an insulated body of an appropriate shape such as a cylindricalcanister shaped housing having a cavity therein which defines avaporizing chamber, a conduit means such as tubing for feeding apropellant liquid at propulsion system feed pressure from a storagecontainer into the housing and thus into the vaporizing chamber and thenexhausting the propellant vapor from the chamber, a porous mediumdisposed in the chamber and positioned so as to block free flow ofpropellant through the chamber, and a heating means such as a resistanceheater positioned within the housing and communicating with the mediumfor controllably heating the liquid propellant in the medium in thevaporizing chamber.

The porous medium has a very low porosity and liquid permeability so asto limit the maximum mass flow rate through the medium to less thanabout 2 lbm/hr. The vapor permeability of such a material iscorrespondingly much lower. A ceramic material such as alumina is anexample of a porous material for the vaporizing medium. The thermalenergy from the resistance heater raises the temperature of the liquidpropellant in the porous medium to establish a variable liquid/vaportransition zone within the medium. This transition zone is responsive tothe amount of heat transmitted from the heater. The position of thetransition zone, i.e. its spacing from the heater, varies directly withthe temperature within the porous medium and thus the heat input fromthe heater. In addition, the zone position determines the overall flowimpedance of the vaporizing chamber. If the transition zone is near theinlet of the medium, furthest from the heat source, a "vapor lock" iscreated which reduces the mass flow of propellant to near zero. If thetransition zone is at or near the outlet of the medium, nearest to theheat source, flow of propellant through the medium is maximum.

One preferred embodiment of the present invention includes a cylindricalcanister having an outer wall and at least two concentrically spacedannular inner walls which define a vaporizing chamber therebetween.Disposed between the inner walls is a porous medium having a very lowporosity and very low permeability. The medium may be a self supportingsolid or may require an external support structure such as the walls tohold it together. If the porous medium is a structurally solid body andpreferably in the shape of a tube, these annular inner walls may beeliminated. If the inner walls are present, the inner walls will haveapertures therethrough for passage of the propellant into and out of thevaporizing chamber.

A liquid propellant from a conventional pressurized propellant supply isfed via tubing through the outer canister wall into the annular spacebetween the outer inner wall and the outer wall. The propellant can thenpass through the apertures in the outer inner wall into the vaporizingchamber and the porous medium. The propellant passes out of thevaporizing chamber through the apertures in the inner wall into acentral axial passage and then out of the canister through anotherconduit or tube which then directs the vaporized propellant to thethruster.

The porous medium is selected from materials that have a low liquidpermeability and must have a permeability that is effective to limitpassage of liquid propellant therethrough. Due to the small pore sizes,surface forces will promote good contact between the heating surfaces ofthe porous medium and the liquid propellant so that the solid/liquidheat transfer coefficient is high.

Centrally disposed along the axis of the canister in this embodiment isa heating element such as a cylindrical resistance heating element. Thisheating element may also have radial ribs to conduct heat from theheating element to the inner wall of the vaporizing chamber. Heat isalso transferred by convection and radiation from the resistance heateracross the inner wall to the medium in the vaporizing chamber.

The liquid/vapor transition zone referred to above is created first nearthe inside surface of the medium as the current to the resistance heateris increased. As additional heat flux is supplied to the medium, thistransition zone moves radially outward, vaporizing more of the liquidpropellant within the medium and creating a resistance or more preciselya back pressure against the flow of the liquid propellant through themedium. Eventually, when the zone approaches the outer margin of themedium, the back pressure creates a "vapor lock" condition whicheffectively terminates flow through the medium. Conversely, as thetemperature of the heating element and thus the temperature gradientacross the porous medium decreases, the transition zone moves radiallyinward thus increasing the mass flow rate of liquid propellant and vaporthrough the medium in the vaporizing chamber and thus through thecanister.

In other words, the liquid/vapor transition zone within the porousmedium is responsive to the amount of heat added by the resistanceheating element. The location of this transition zone will be determinedby an overall energy balance on the porous medium itself. Generallyspeaking, energy enters the porous medium as conducted and radiatedenergy from the heater or as sensible heat of the incoming fluid, andexits either as sensible or latent heat in the vapor, or as radiated andconducted heat losses from the outer walls of the device. The positionand extent of the transition zone effectively controls the mass flowrate of propellant through the vaporizing chamber. This method ofpropellant flow control is particularly well adapted to the low flowrates required for proper operation of low thrust arcjet thrusters andresistojets in the space environments where the effects of gravity areminimal or nonexistent.

The porous medium in the apparatus in accordance with the presentinvention provides good contact between the heating surface (the porousmedium itself) and the propellant liquid being vaporized. It has verysmall pores and therefore a large surface area for fluid contact. Thesolid/liquid heat transfer coefficient is therefore quite high. In a lowporosity material chosen to give the appropriate solid/liquidinterfacial tension, surface forces dominate and promote intimatesolid/liquid contact, thus ensuring efficient heat transfer.

The apparatus of the invention may also act simply as a flow restrictor.Flow restrictors or fluid resistors of some kind are typically installedupstream of small electrical and chemical thrusters in spaceapplications to promote stable fluid dynamic operation. The apparatus inaccordance with the present invention also provides the necessary highimpedance to liquid flow and so may also be used for this purpose inaddition to use as a vaporizer.

The apparatus in accordance with the present invention can also bedesigned such that a portion of the medium, preferably an annular porousring, is always upstream of the liquid/vapor transition. The liquidpermeability of this region may also be specified to be low enough thatthe pressure drop across it will be quite high. This outermost portionof the porous medium can thus serve as an intrinsic fluid resistor. Inconjunction, a second, inner, porous ring, having a higher vaporpermeability, can afford finer flow rate control if the vapor/liquidtransition zone is kept within this inner ring.

In another embodiment of the invention, multiple heaters may be embeddedin the porous medium to selectively vaporize portions of the liquid inthe medium. In this way a variable resistance flow restrictor may becreated with or without the exiting fluid being a vapor.

Other features and advantages of the present invention will becomeapparent from a reading of the following detailed description when takenin conjuction with the appended claims and the accompanying drawing. Inthe several embodiments shown in the figures of the drawing, likenumbers are used to describe like components and to simplify thedescription.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal sectional view through a first preferredembodiment of the present invention.

FIG. 2 is sectional view taken on the line 2--2 in FIG. 1.

FIG. 3 is a longitudinal sectional view through a second preferredembodiment of the present invention.

FIG. 4 is a sectional view taken on the line 4--4 in FIG. 3.

FIG. 5 is a longitudinal sectional view through a third preferredembodiment of the present invention.

FIG. 6 is sectional view taken on the line 6--6 in FIG. 5.

FIG. 7 is a longitudinal sectional view through a fourth preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first preferred embodiment of the propellant vaporizing apparatus inaccordance with the present invention is illustrated in FIG. 1. Thepropellant vaporizing apparatus 10 is preferably a cylindrical canisterbody having an outer wall 12, a concentric intermediate wall 14, and acoaxial concentric inner wall 16 defining an annular vaporizing chamber15 therebetween coaxial with a central axis 18. A resistive heater 20 iscentrally disposed along axis 18 inside and spaced from inner inner wall16. The power to the heater 20 could be variably controlled by anyconventional means.

A ring shaped porous medium 22 effectively divides chamber 15 into twoportions, an inlet portion 17 and an outlet portion 19. As shown in FIG.1, the medium 22 preferably fills both portions of the vaporizingchamber 15 and is retained between the intermediate wall 14 and theinner wall 16.

Alternatively, if the porous medium is a structurally self supportingsolid body, walls 14 and 16 may be eliminated as in the otherembodiments described below. In such cases, the support function of thewalls would be performed by the porous medium 22 itself.

Annularly enclosing the intermediate wall 14 is an annular liquid feedchannel 24 which communicates with a propellant inlet tubing 26. Theinner wall 16 surrounds and defines a central axial vapor header 28which communicates with an exhaust tube 30 which is in turn connected toany conventional electrothermal thruster such as an arcjet or resistojet(not shown). Finally, interposed between the liquid propellant channel24 and outer wall 12 may be a layer of insulation 31 to insulate thevaporizing apparatus 10 from the outside space environment.

In operation, a liquid propellant such as water or ammonia is fed indirection 32 under pressure via the tubing 26 into the annular channel24 and through apertures 34 and into the porous medium 22. The porousmedium 22 is preferably a material such as alumina (aluminum oxide) hasa low porosity and thus a low liquid permeability. The propellant thenpermeates, as liquid, into the porous medium 22, is vaporized therein,and exits as vapor through apertures 36 in inner wall 16 into the vaporheader 28. The propellant vapor then exhausts through the propellantvapor exhaust tubing 30.

The maximum propellant mass flow rate through the apparatus 10, lessthan about 2 lbm/hr, is obtained when the liquid/vapor transition occursat the inner surface of the porous annular ring, i.e. when there is pureliquid flowing through the porous medium 22. If no current is suppliedto the heater 20, pure liquid flows out the exhaust tube 30 and thevaporizer acts simply as a flow restrictor with its flow resistancedetermined by the porosity of the medium 22.

Normally, current is supplied to the heater 20 to vaporize thepropellant at least in vapor header 28. The propellant liquid whichpasses through medium 22 is vaporized as it enters through apertures 36into the vapor header 28 by the heater 20. At this point flow throughthe vaporizing apparatus 10 is maximum or very near maximum. As thepower supplied to heater 20 is further increased, a annular liquid/vaportransition zone 38 is established within the porous medium 22. Stillfurther heat addition causes this transition zone 38 to move radiallyoutward. The position of this transition zone is determined by anoverall heat balance on the vaporizing apparatus 10. The use of thistype of flow control in accordance with the invention will permitobtaining an accuracy of control within about ±0.01 lbm/hr.

As the permeability of the porous medium 22 is much lower for thepropellant vapor, propellant flow through the porous medium may beclosely controlled by the radial position of the liquid/vapor transitionzone 38 and thus by the power supplied to the heater. As noted above,the mass flow rate is maximum when the transition zone is locatedadjacent the inner wall 16. The propellant flow is minimum, effectivelynegligible, when the liquid/vapor transition zone is adjacent theintermediate wall 14.

The overall liquid permeability of the porous medium 22, whichpreferably has an annular cylindrical ring shape, is a function of itsspecific liquid permeability, thickness, inner and outer radii, andlength, and therefore will be determined by the 100 percent flow ratedesign point specified for the device. The position of the liquid/vaportransition zone will be determined by a complex interaction of theradial thermal and pressure gradients within the porous ring. Ingeneral, the fluid pressure will decrease with decreasing radius withinthe ring while the temperature generated in the medium 22 by the heater20 will increase with decreasing radius. Ideally, vaporization will thenoccur at that radius within the ring for which the vapor pressure of theliquid is equal to the fluid pressure. The vapor pressure of the liquidmay be appreciably affected by the radius of curvature in thisembodiment of the vapor/liquid interface due to the small pore sizes ofthe porous material.

As the heater power is increased, the liquid/vapor transition zone 38within the medium 22 will move to a larger steady state radius, and themass flow rate for the device will decrease. As the heater power isincreased, the specific enthalpy of the exit stream will also increase.Since substantial additional superheat will be added in any casedownstream of the vaporizer in any electrothermal propulsionapplication, this increased superheat would not be lost.

The basic concept of controlling the flow through the propellantvaporizer by varying the location of the liquid/vapor transition zone isnot limited to the configuration shown in FIGS. 1 and 2. Flow control isaccomplished because, for a given pressure drop, the vapor phase massflow rate through a given flow restriction will be much lower than theliquid phase mass flow rate through the same restriction. Accordingly,other physical configurations and embodiments of the basic concept ofthe vaporizer flow control apparatus are possible and are envisionedwithin the scope of the present invention, including, but not limited tothe following variations.

The porous medium may be designed to have radially variablepermeability. For example, porous medium 22 may have a permeabilitywhich is a decreasing function of the ring radius as in the secondembodiment shown in FIGS. 3 and 4.

The vaporizer 40 comprises a cylindrical canister having an outer wall12 and defining therein a vaporizing chamber 15 as in the previousembodiment. A resistive heater 20 is centrally disposed along axis 18 inthe center of the porous medium 22. The porous medium 22 is a tubularsolid positioned in the chamber 15 so as to create an outer annularliquid feed channel 24 and a central axial vapor header 28 around theheater 20 as in the previous embodiment. An inlet tube 26 feedspropellant 32 into the feed channel 24 and an exhaust tube 30 directsthe propellant vapor 32 from the vapor header 28 to the thruster (notshown). A layer 31 of insulation separates the vaporizing chamber 15from the outer wall 12 as in the first embodiment.

In this second preferred embodiment, porous medium 22 is a structurallysolid annular cylinder made up of a plurality of concentric annularportions 22a, 22B, 22C, and 22D and thus walls 14 and 16 are not needed.The outer portion 22A has a lower permeability than the inner portions22B, 22C, and 22D with inner portion 22D having the highestpermeability.

In this embodiment, the outer portion 22A acts primarily as a liquidflow restrictor and the transition zone 38 may range radially over allfour portions depending on the current input to the heater. Thisarrangement can provide very fine flow control. In another variation ofthis embodiment, medium 22 could be a single solid body having aradially varying porosity. Thus, the outer portion of the ring wouldhave a relatively low permeability. Conversely, the inner portion of thering, within which the vapor/liquid transition zone would range, wouldhave a much higher permeability. This would also allow even finercontrol of mass flow rate.

A third preferred embodiment of the present invention is illustratedschematically in FIGS. 5 and 6. The vaporizer 60 again comprises acylindrical canister having an outer wall 12 and defining therein avaporizing chamber 15 as in the previous embodiment. The porous medium62 is a tubular solid positioned in the chamber 15 so as to create anouter annular liquid feed channel 24 and a central axial vapor header 28as in the previous embodiment. An inlet tube 26 feeds propellant 32 intothe feed channel 24 and an exhaust tube 30 directs the propellant 32from the vapor header 28 to the thruster (not shown). A layer 31 ofinsulation separates the vaporizing chamber 15 from the outer wall 12 asin the first embodiment.

However, in this embodiment, the heater is not centrally located in thevapor header, but is made up of multiple heater elements 64 embedded inthe porous medium 62 and radially spaced about the vapor header 28. Inthis embodiment the propellant 32, or other liquid to be throttled orvaporized, is not necessarily entirely vaporized in the chamber 15. Thearrangement of heaters embedded within the medium 22 permits selectiveenergization of individual heaters 64 to create localized vapor pocketsin the adjacent porous medium 62 which produces a variable back pressureagainst the flow of fluid through the medium 62. In this manner theoverall flow rate through the vaporizer 60 can be controlled withoutnecessarily converting the exiting fluid 32 to a vapor.

A fourth preferred embodiment of the vaporizer in accordance with thepresent invention is shown in FIG. 7. The vaporizer 70 includes acylindrical canister body 12 defining a vaporizing chamber 15 therein asin the first and second embodiments. A resistive heater 20 is centrallydisposed along axis 18 in the center of the vaporizing chamber 15. Thetubular solid medium 72 is positioned in the chamber 15 so as to createdisk shaped liquid inlet feed channel 24 at one end of the medium 72 anda disk shaped vapor header 28 at the other end of the tubular medium. Aninlet tube 26 feeds a propellant fluid in direction 32 into the feedchannel 24 and an exhaust tube 30 directs the propellant vapor from thevapor header 28 to the thruster (not shown). A layer 31 of insulationseparates the vaporizing chamber 15 from the outer wall 12 as in thefirst embodiment.

However, in this embodiment, the medium 72 is made up of a bundlearrangement or bed of generally axially aligned capillary tubes 74around the heater 20. The bundle of capillaries 74 functions the same asthe porous medium of the previous embodiments. The radii of the tubes 74are sufficiently small so as to ensure that surface forces dominate inthe force balance and thus good thermal contact is provided between thecapillary tube walls and the fluid propellant. The amount of heatproduced by the heater 20 determines the location of the liquid/vaportransition zone 38 within the capillary tube medium 72 and thus the backpressure against the flow of fluid through the vaporizer.

In any of these embodiments, non-cylindrical geometries couldalternatively be utilized. Proper placement of heaters for example,embedded within the medium, as in FIGS. 5 and 6, to control the vaporphase flow resistance would allow application of the basic idea in avariety of configurations or geometries. For example, radially outwardrather than inward flow in a cylindrical geometry might offer some fluiddynamic advantages. The fundamental approach is also applicable in axialand spherical geometries, as well.

Other variations could include the utilization of multiple heaters as inFIGS. 5 and 6 embedded in or positioned adjacent the porous or capillarytube bed in these different geometries. The heater means could also bepositioned around the outside radius of the vaporizing chamber and thepropellant inlet along the central axis. The fluid flow direction insuch an apparatus would be radially outward, with the vapor headerpositioned to collect exiting vapor around the outer surface of thevaporizing porous medium.

The overall shape may be spherical or any other shape depending on theenergy balance requirements or overall system requirements. Thecontrolled regional vapor pocket approach could be utilized to reducedrastically the mass flow rate through portions of a packed, porous, ortubular capillary bed without altering the phase of the fluid exitingthe device. Although stationery pockets of vapor phase would existwithin such a device during steady state operation, the exit streamcould in fact be liquid.

While the invention has been described above with reference to specificembodiments thereof, it is apparent that many changes, modifications andvariations can be made without departing from the inventive conceptdisclosed herein. Accordingly, it is intended to embrace all suchchanges, modifications and variations that fall within the spirit andbroad scope of the appended claims. All patent applications, patents andother publications cited herein are incorporated by reference in theirentirety.

What is claimed is:
 1. A method of controlling a flow of a fluid comprising the steps of:a) passing a liquid phase of said fluid into a porous medium; b) producing a liquid/vapor transition zone within said medium so as to create a back pressure against flow of fluid through said medium; c) providing an electrical heating means coacting with said medium for controllably raising the temperature of said fluid in said medium; and d) controlling the position of said zone in said medium by controlling the power into said heating means; and e) allowing said fluid to exit said medium.
 2. A method of controlling a flow of a fluid comprising the steps of:a) passing a liquid phase of said fluid into a porous medium; b) producing a liquid/vapor transition zone within said medium so as to create a back pressure against flow of fluid through said medium; c) controlling the size of said zone to vary the back pressure; and d) allowing said fluid to exit said medium.
 3. A method of controlling a flow of a fluid comprising the steps of:a) passing a liquid phase of said fluid into a porous medium; b) producing a liquid/vapor transition zone within said medium so as to create a back pressure against flow of fluid through said medium; c) controlling the location of said zone to vary the back pressure; and d) allowing said fluid to exit said medium. 